The present invention relates to apparatus and techniques for optically determining the clearance between at least one translating member and a relatively stationary second member, and more particularly, to such clearance determination between the radial edge of a rotating blade member and a shroud in a gas turbine engine.
Modern aircraft gas turbine engines typically employ high pressure ratio compressors and turbine stages. This engine design reduces weight and improves overall efficiency, but is more sensitive to the blade tip-to-shroud clearance. In this connection, unnecessarily large clearances cause excessive leakage and also tend to induce aerodynamic losses within and downstream of the particular blade tip-to-shroud position. Accordingly, it is well known that significant advantages in engine performance and engine life extension result when a clearance control technique is implemented. In addition, in the development of an aircraft gas turbine engine, a designer needs to identify the effect of clearance on performance and life, and hence, needs running clearance measurements during developmental engine testing.
Various clearance measuring techniques have been employed in the past and have had some success. However, each of the known measuring techniques has several disadvantages associated with its use. For example, one measuring technique employs blade rub pins in the form of small metal projections mounted in the shroud. These rub pins wear away upon blade contact, and hence, the engine must be disassembled in order to find the final pin length. Also, such rub pins only show the minimum clearance encountered such that the designer is not able to discern the time or particular engine parameters when the minimum clearance event occurred. In view of the foregoing, the use of blade rub pins is not considered practical in an operating gas turbine engine of the type which employs clearance control techniques. Another clearance measuring technique employs high energy X-rays. However, this technique involves undesirable accuracy limitations and is usable generally only on shrouded blades. Further, use of such X-rays requires specially equipped engine test sites. Another clearance measuring technique employs touch probes for measuring the clearance to the highest blade, i.e., the blade having the minimum clearance. Such touch probes cannot measure rotor orbiting, i.e., the amount of out-of-roundness or out-of-centerness, and the touch probes cannot measure clearance changes during fast engine transients. Capacitance clearance measuring techniques have also been employed wherein a probe senses capacitance which is representative of blade tip clearance. However, the accuracy of such capacitance clearance techniques is detrimentally affected by several mechanisms. For example, any change in stray capacitances or in the blade tip shape, e.g., dirt buildup on blade sides, formation of melt, affects the capacitance sensed. Similarly, normal manufacturing tolerances with respect to individual blade tip thickness also affect the technique's accuracy.
Another clearance measuring technique employs optical devices, such as optical probes, and triangulation to determine blade tip location. However, a difficulty with one such optical measuring technique is due to the conventional optical probe typically employed. Such optical probes are generally characterized by undesirably high internal light reflections within the probe. Also, these optical measuring techniques have heretofore exhibited limited responsiveness and accuracy. For example, one such optical measuring technique employs closed circuit television, resulting in an inability to determine individual blade clearances in a rapidly rotating blade structure.
Accordingly, it is a general object of this invention to provide improved optical apparatus and techniques for determining the clearance between the radial edge of at least one rotating blade member and a relatively stationary second member.
Another object of the present invention is to provide such apparatus and techniques wherein high internal light reflections within the probe are reduced.
Another object of the present invention is to provide such apparatus and techniques capable of developing a substantially instantaneous representation of the clearance between the radial edge and the second member.